Re: [PATCH -mm 5/7] add user namespace

[Cedric Le Goater <clg@fr.ibm.com>]

Kirill Korotaev wrote:
>>> Lets take a look at sys_setpriority() or any other function calling
>>> find_user():
>>> it can change the priority for all user or group processes like:
>>>
>>> do_each_thread_ve(g, p) {
>>>   if (p->uid == who)
>>>       error = set_one_prio(p, niceval, error);
>>> } while_each_thread_ve(g, p);
>>
>>
>> eh. this is openvz code ! thanks :)
> it doesn't matter :)

it does. it means for me that you are studying proposals to see how if fits
with your existing code. which is good.

> 2.6.17 code is:
>                        do_each_thread(g, p)
>                                if (p->uid == who)
>                                        error = set_one_prio(p, niceval,
> error);
>                        while_each_thread(g, p);
> 
> when introducing process namespaces we will have to isolate processes
> somehow and this loop, agree?

yes

> in this case 1 user-namespace can belong to 2 process-namespaces, agree?
> how do you see this loop in the future making sure that above situation
> is handled correctly?

IMO, the loop should apply to the current->pidspace or equivalent inside
the loop

> how many other such places do we have?

if it's embedded in the loop, it should not be too much of an issue ?

>>> which essentially means that user-namespace becomes coupled with
>>> process-namespace. Sure, we can check in every such place for
>>> p->nsproxy->user_ns == current->nsproxy->user_ns
>>> condition. But this a way IMHO leading to kernel full of security
>>> crap which is hardly maintainable.
>>
>> only 4 syscalls use find_user() : sys_setpriority, sys_getpriority,
>> sys_ioprio_set, sys_ioprio_get and they use it very simply to check if a
>> user_struct exists for a given uid. So, it should be OK. But please
>> see the attached patch.
>
> the problem is not in find_user() actually. but in uid comparison inside
> some kind of process iteration loop. In this case you select processes
> p which belong to both namespaces simultenously: i.e. processes p which
> belong both to user-namespace U and process-namespace P.
> 
> I hope I was more clear this time :)

yes thanks,

for the moment, if processes are not isolated in some others ways, like in
openvz, these kind of loops would need the extra test 'p->nsproxy->user_ns
== current->nsproxy->user_ns' on user namespace to be valid. same issue for
filesystem and many other places. eric raised that point.

In theory, if I understand well eric's concept of namespaces, a task
belongs to a union of namespaces : ipc, process, user, net, utsname, fs,
etc. some of these namespaces could be default namespaces and some not
because they were unshared in some way: clone, unshare, exec, but in a safe
way.

They are necessary bricks for a bigger abstraction, let's call it
container, but they not sufficient by them selves because they have
dependencies. The container comes as a whole and not subsystem by
subsystem, I agree with you on that point.

>>> Another example of not so evident coupling here:
>>> user structure maintains number of processes/opened
>>> files/sigpending/locked_shm etc.
>>> if a single user can belong to different proccess/ipc/... namespaces
>>> all these becomes unusable.
>>
>>
>> this is the purpose of execns.
>>
>> user namespace can't be unshared through the unshare syscall().
>
> why? we do it fine in OpenVZ.

probably because you use the full container approach in openvz and start
the container by running init ? namespaces are a bit more painful ... I agree.

I'm still struggling with the limits of that namespace concept. Hopefully,
we meet next week because I'm also reaching my limits of digital
interaction on this topic :)

thanks,

C.

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